Method to obtain a temperature coefficient-compensated output signal in an optical current measuring sensor

ABSTRACT

In a magneto-optical sensor with −/+ intensity norming, the temperature-coefficient-compensated output signal (S) is obtained by formation of a quotient of the alternating portion (P AC ) of the intensity-normed signal (P) and a function (f(P DC , P ACeff )), determined by a calibration measurement, of a direct portion (P DC ) of the intensity-normed signal (P) and of a root-mean-square value (P ACeff ) of the alternating portion (P AC ). A corresponding method for sensors with AC/DC intensity norming is indicated.

This application is a 371 of PCT/de97/01854, filed Aug. 26, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for obtaining atemperature-coefficient-compensated output signal in an optical sensorfor measuring a periodically fluctuating electrical and/or magneticfield strength.

2. Description of the Related Art

Optical sensors of the cited type are known, in particular in the formof current measurement sensors for measuring the current strength of analternating current conducted in a conductor, which produces in theimmediate environment of the conductor an electromagnetic field with anelectrical and magnetic field strength that fluctuates periodicallycorresponding to the current strength of the alternating current. Thesensor measures, for example according to the Faraday effect, theperiodically fluctuating magnetic field strength, according to which thecurrent strength can be inferred.

Voltage measurement sensors operate in a similar manner for themeasurement of an alternating voltage applied to a conductor, whichvoltage produces in the immediate environment of the conductor anelectrical field with an electrical field strength that fluctuatesperiodically corresponding to the alternating voltage. This sensormeasures, for example according to the Pockels effect, the periodicallyfluctuating electrical field strength, according to which thealternating voltage can be inferred.

The respective field strength is measured in that light that can beinfluenced by the field strength is sent through the periodicallyfluctuating field, and from this light that has been sent through andinfluenced two intensity signals are produced that are separate from oneanother and that comprise intensities containing intensity portions thatfluctuate in chronologically periodic fashion in phase opposition to oneanother dependent on the field strength of the periodically fluctuatingfield.

The influencing of the light by the field can be based on various knownphysical effects, for example the cited Faraday or Pockels effect. As anexample, the Faraday effect, which is frequently used in currentmeasurement sensors, is explained in more detail, in which linearlypolarized light from a polarizer is influenced by the magnetic fieldstrength in such a way that the polarization plane of the polarizedlight is rotated, dependent on the field strength, in relation to thepolarization plane determined by the polarizer, or in correspondinglyperiodically fluctuating fashion in the case of the periodicallyfluctuating field strength. The linearly polarized light which has beeninfluenced by the magnetic field strength can be supplied to ananalyzer, which forms two polarized light portions that areperpendicular to one another from this light, whose intensities relativeto one another depend, in phase opposition, on the polarization state ofthe field-strength-influenced light in relation to the polarizationplane determined by the polarizer, and which supplement each other toform an overall intensity that is independent of the angular position ofthe polarization plane of the field-strength-influenced light. In thiscase, these light portions form the two intensity signals that indicatethe periodically fluctuating field strength [sic].

Such intensity signals can be obtained in a similar manner in voltagemeasurement sensors which for example exploit the Pockels effect.

With the aid of the variable derived from the two intensity signals,which corresponds to the quotient of the difference of the intensitiesof the two intensity signals and the sum of these intensities and thatincludes the direct portion and the periodically fluctuating alternatingportion, to which a root-mean-square value can be allocated, the outputsignal of such a sensor, indicating the measured field strength andtherewith the current strength or voltage, is obtained.

This output signal, in particular a periodically fluctuating portioncontained therein, often exhibits a temperature drift whose cause isgrounded in the physical effect on which the measurement is based,and/or in disturbances such as for example mechanical stresses and/or alinear double refraction.

In order to remove this problem, for example a temperature coefficientcompensation method is proposed in relation to a magneto-optical sensorbased on the Faraday effect, in which the two intensity signals areobtained in the manner described above with the aid of linearlypolarized light from a polarizer, for example a laser diode, and anoptical analyzer. In this known temperature compensation method, it isnecessary to adjust the polarizer and analyzer very precisely to oneanother with respect to their angular setting, i.e., in such a way thatthe polarization plane determined by the polarizer and the polarizationplane determined by the analyzer stand as precisely as possible at anangle of 45° to one another.

SUMMARY OF THE INVENTION

The present invention provides a new type of method for obtaining atemperature-coefficient-compensated output signal, having the advantagethat in a sensor of the cited type, if the two intensity signals areobtained with the aid of a polarizer and analyzer, larger angularmisadjustments are permissible between the polarizer and the analyzer incomparison to the known temperature compensation method.

The method, in which the quantity is derived from the two intensitysignals, which quantity corresponds to a quotient of a difference of theintensities of the two intensity signals and the sum of theseintensities, is known as −/+ intensity norming.

The basic idea of the invention underlying the inventive method permits,but is not limited to, this intensity norming, but rather can also beapplied analogously to sensors in which what is known as AC/DC intensitynorming is present, i.e., in sensors.

The invention likewise provides a new type of method for obtaining atemperature-coefficient-compensated output signal, with the advantagethat in a sensor of the named type, if the two intensity signals areobtained with the aid of a polarizer and analyzer, greater angularmisadjustments are permissible between the polarizer and the analyzer incomparison to the known temperature compensation method.

In the inventive method, angular misadjustments between the polarizerand the analyzer of more than 5° advantageously adversely affect thefunction of the inventive method only inessentially in the sensoritself. The reduction of sensitivity is low and is proportional to thecosine of the faulty angle between the polarization plane of thepolarizer and that of the analyzer. This advantageously simplifies thedesign and manufacture of the sensor in which the inventive method isapplied.

However, the inventive method is not limited to sensors that operatewith polarizer and analyzer, but rather can generally be applied also inother sensors, and in this way extends the technology.

The function to be used in the inventive method and predetermined by thecalibration measurement can be approximated by a polynomial of apredeterminable degree and/or can be stored in approximate form in alookup table.

A preferred arrangement for the execution of a method for obtaining atemperature-coefficient-compensated output signal in an optical sensorfor measuring a periodically fluctuating electrical and/or magneticfield strength, in which sensor

light that can be influenced by the field strength is sent through theperiodically fluctuating field strength,

from this light that has been sent through and influenced, two intensitysignals are produced that are separate from one another and thatcomprise intensities containing intensity portions that fluctuate inchronologically periodic fashion in phase opposition to one anotherdependent on the periodically fluctuating field strength, and

a quantity is derived from the two intensity signals, which quantitycorresponds to a quotient of a difference of the intensities of the twointensity signals and the sum of these intensities, whereby

the quantity comprises a direct portion and a periodically fluctuatingalternating portion with a root-mean-square value, in that thetemperature-coefficient-compensated output signal is obtained byformation of a quotient of the alternating portion of the derivedquantity and a function, determined by a calibration measurement, of thedirect portion and of a root-mean-square value of the alternatingportion of the derived quantity, this being executed by

a means for obtaining the direct portion of the derived quantity,

a means for obtaining the alternating portion of the derived quantityand formation of the root-mean-square value of this portion, and

a means for forming the quotient of the direct portion of the derivedquantity and the function) of the direct portion and of theroot-mean-square value of the alternating portion of the derivedquantity. A preferred arrangement for the execution of a method forobtaining a temperature-coefficient-compensated output signal in anoptical sensor for measuring a periodically fluctuating electricaland/or magnetic field strength, in which sensor

light is sent through the periodically fluctuating field strength,

from this light that has been sent through, two intensity signals areproduced comprising intensities that contain alternating portions thatfluctuate periodically in phase opposition to one another dependent onthe periodically fluctuating field strength and that contain arespective direct portion, and

a respective normed quantity is derived from each of the two intensitysignals, which normed quantity corresponds to a quotient of thealternating portion and the direct portion of the intensity of thisintensity signal, in that from the two derived quantities

a first quotient is derived from the sum and a difference of the twonormed quantities,

a periodically fluctuating second quotient is derived from the twofoldproduct and the difference of the two normed quantities, and

the temperature-coefficient-compensated output signal is obtained

by formation of a quotient of the periodically fluctuating secondquotient and a function, determined by a calibration measurement, of thefirst quotient and of a root-mean-square value of the periodicallyfluctuating second quotient, this being executed by

a means for formation of the first quotient from the sum and thedifference of the two derived normed quantities,

a means for formation of the periodically fluctuating second quotient ofthe twofold product and the difference of the two derived normedquantities and of the root-mean-square value of the second quotient,

a means for formation of the quotient of the formed periodicallyfluctuating second quotient and the function, determined by thecalibration measurement, of the formed first quotient and of aroot-mean-square value of the periodically fluctuating second quotient.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following specification, the invention is explained in moredetail by way of example on the basis of the figures.

FIG. 1 shows, in a schematic representation, an output part, whichproduces the two intensity signals and the quantity derived therefrom,of an optical sensor with −/+ intensity norming, and an arrangement,coupled to this output part, for the execution of the method forobtaining a temperature-coefficient-compensated output signal in thissensor,

FIG. 2 shows, in a schematic representation, an output part, whichproduces the two intensity signals and the two normed quantities derivedtherefrom, of an optical sensor with AC/DC intensity norming, and anarrangement, coupled to this output part, for execution of a method forobtaining a temperature-coefficient-compensated output signal in thissensor,

FIG. 3 shows a diagram which shows the percent deviation of thenon-temperature-coefficient-compensated output signal of amagneto-optical sensor from its ideal value, and

FIG. 4 shows a diagram showing the percent deviation of the outputsignal, corrected with the inventive method, of the magneto-opticalsensor given an angular misadjustment of 5° between the analyzer and thepolarizer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the examples according to FIGS. 1 and 2, it is assumed that theoptical sensor is a magneto-optical or electro-optical sensor, asdescribed in more detail above, which measures the periodicallyfluctuating magnetic and/or electrical field strength of a magneticand/or electrical field produced by a current or a voltage, e.g.exploiting the Faraday or Pockels effect.

Light L with a particular polarization p, produced by a polarizer (notshown), is sent through the periodically fluctuating field, whichinfluences the polarization dependent on the magnetic or electricalfield strength in such a way that after passing through the field thelight L comprises a polarization p′ that is modified in relation to theparticular polarization produced by the polarizer. Because the magneticor electrical field or, respectively, the field strength thereofperiodically fluctuates, the polarization p′ of the polarized light thathas passed through this field also fluctuates periodically in relationto the polarization determined by the polarizer.

In the following, let it be assumed without limitation of generalitythat the sensor is a current measurement sensor exploiting the Faradayeffect. In this case, the polarizer (not shown) produces light L with adetermined linear polarization p, which is sent through an opticalmedium with a Verdet constant arranged in the periodically fluctuatingmagnetic field, for example a glass fiber coil surrounding acurrent-conducting conductor.

The periodically fluctuating magnetic field influences the linearlypolarized light L in such a way that after passage through the field thelinear polarization p′ periodically fluctuates in relation to thedetermined linear polarization. The light L [. . . ] fluctuatingpolarization p′ is supplied to an analyzer 9 with a polarization planethat is set as precisely as possible at an angle of 45° to thepolarization plane of the determined polarization p of the linearlypolarized light L produced by the polarizer, for example a laser diode.

From the light L with the fluctuating linear polarization p′, theanalyzer 9 produces

an intensity signal L with a linear polarization p1 with a fixedpolarization plane and an intensity I1, containing an intensity portionthat fluctuates periodically dependent on the periodically fluctuatingfield strength, and

an intensity signal L2 of a linear polarization p2 with a fixedpolarization plane perpendicular to the fixed polarization plane of theintensity signal L1 and with an intensity I2 that contains an intensityportion that fluctuates periodically dependent on the periodicallyfluctuating field strength, whereby

the periodically fluctuating intensity portions of intensities I1 or,respectively, I2 of the two intensity signals L1 and L2 fluctuateperiodically in phase opposition to one another, dependent on theperiodically fluctuating field strength.

An example of such an analyzer 10 is for example a Wollaston prism,which is often used in such current measurement sensors.

In the example according to FIG. 1, the quantity P is derived from thetwo intensity signals L1 and L2, corresponding to a quotient of, forexample, the difference I1−I2 of the intensities I1 and I2 of the twointensity signals L1 and L2 and the sum I1+I2 of these two intensitysignals I1 and I2.

For the formation of this derived quantity P=(I1−I2)/(I1+I2), the means10 in FIG. 1 is provided, which preferably comprises a digital processorand digitally processes the supplied intensity signals L1 and L2,although an analog processing is equally possible.

The derived quantity P characterizes the sensor as the sensor with −/+intensity norming.

The derived quantity P naturally contains a direct portion P_(DC) and analternating portion P_(AC) that fluctuates periodically corresponding tothe magnetic field strength to which alternating portion aroot-mean-square value P_(ACeff) can be allocated.

For the inventive method in relation to the sensor with −/+ intensitynorming, these quantities P_(DC), P_(AC) and P_(ACeff) are decisive.

Accordingly, from the derived quantity P in the means 11 in FIG. 1 thedirect portion P_(DC) thereof can be obtained, while in the means 12 thealternating portion P_(AC) of the derived quantity P is obtained and theroot-mean-square value P_(ACeff) of this portion P_(AC) is formed.P_(DC) can for example be obtained by means of low-pass filtering, andP_(AC) can for example be obtained by high-pass filtering or formationof P−P_(DC).

The means 12 can for example be fashioned in such a way that a means 12₁ is provided that obtains the alternating portion P_(AC) from thederived quantity P, which alternating portion is supplied to a separatemeans 12 ₂, in which the root-mean-square value P_(ACeff) is formed.

According to the invention, the temperature-coefficient-compensatedoutput signal S is obtained by formation of the quotient

S=P _(AC) /f(P _(DC) , P _(ACeff))

from the alternating portion P_(AC) of the derived quantity P and thefunction f (P_(DC), P_(ACeff)) determined by the calibration measurementin the desired temperature range, of the direct portion P_(DC) and ofthe root-mean-square value P_(ACeff) of the alternating portion P_(AC)of the derived quantity P.

Given a digital signal processing, from the value at a particularsampling point of the direct portion P_(DC) and of the root-mean-squarevalue P_(ACeff), the function value belonging to this sampling point ofthe predetermined function f(P_(DC), P_(ACeff)) is determined, and fromthis the value belonging to this sampling point of the output signal Sis formed by formation of the quotient with the value belonging to thissampling point of the alternating portion P_(AC) and the functionalvalue belonging to this sampling point. This method can be repeated fromsampling point to sampling point. For the formation of the functionalvalue belonging to this sampling point, the means 13 is provided, towhich the value belonging to this sampling point of the direct portionP_(DC) and the root-mean-square value P_(ACeff) belonging to thissampling point are supplied, and which outputs to the means 14 thefunctional value, belonging to this value of the direct portion P_(DC)and to this root-mean-square value P_(ACeff) of the predeterminedfunction f (P_(DC), P_(ACeff)), as the functional value belonging tothis sampling point of this function, to which means 14 on the otherhand the value belonging to this sampling point of the alternatingportion P_(AC) is supplied and which forms and outputs from thisfunctional value and this value of the alternating portion P_(AC) thetemperature-coefficient-compensated output signal S belonging to thissampling point.

The means 13 can contain a lookup table in which a functional value ofthe function f is respectively allocated to each pair of valuesconsisting of a value of the direct portion P_(DC) and aroot-mean-square value PAceff of the alternating portion P_(AC). Themeans 13 can also contain a processor that calculates the functionalvalues of the function f allocated to the various value pairs from avalue of the direct portion P_(DC) and a root-mean-square valueP_(ACeff), e.g. in approximated fashion with the aid of a polynomial ofa selectable order.

The value of the direct portion P_(DC) and of the root-mean-square valueP_(ACeff) do not respectively change over a longer time period incomparison to the value of the alternating portion P_(AC), so that inthis respective time period these values can be regarded as constant,and do not need to be determined as often as the value of thealternating portion P_(AC).

The example according to FIG. 2 is based on a sensor with AC/DCintensity norming. In this sensor, according to the invention a normedquantity S₁ or, respectively, S₂ is respectively derived from eachintensity signal L1 and L2 from the analyzer 9. The normed quantity S₁derived from the intensity signal L1 corresponds to the quotient I^(AC)₁/I^(DC) ₁ of the alternating portion I^(AC) ₁ and the direct portionI^(DC) ₁ of the intensity I1 of the intensity signal L1, and thequantity S2 corresponds to the quotient I^(AC) ₂/I^(DC) ₂ of thealternating portion I^(AC) ₂ and the direct portion I^(DC) ₂ of theintensity I2 of the intensity signal L2.

In the example according to FIG. 2, the normed quantity S₁ is formed bythe means 15 ₁, which comprises for example a means 15 ₁₁ for obtainingthe alternating portion I^(AC) ₁, a means 15 ₁₂ for obtaining the directportion I^(DC) ₁ of the intensity signal L1, and a means 15 ₁₃ forforming the quotient I^(AC) ₁/I^(DC) ₁. The normed quantity S₂ is formedby the means 15 ₂, which comprises for example a means 15 ₂₁ forobtaining the alternating portion I^(AC) ₂ of the intensity signal L2, ameans 15 ₂₂ for obtaining the direct portion ^(DC) ₂ I^(DC) ₂ of theintensity signal L2 and a means 15 ₂₃ for forming the quotient I^(AC)₂/I^(DC) ₂.

According to the invention, from the two derived quantities S₁ and S₂the first quotient P′_(DC) is derived from the sum S₁+S₂ and thedifference S₂−S₁ of the two normed quantities S₁ and S₂, i.e.,P′_(DC)=(S₁+S₂)/(S₂−S₁), and the periodically fluctuating secondquotient P_(AC) is derived from the twofold product 2S₁S₂ and thedifference S₂−S₁ of the two normed quantities S₁ and S₂, i.e.P′_(AC)=2S₁S₂/(S₂−S₁), and the temperature-coefficient-compensatedoutput signal S is obtained by formation of the quotientP′_(AC)/f(P′_(DC), P′_(ACeff)) of the periodically fluctuating secondquotient P′_(AC) and the function f(P′_(DC)/ P′_(ACeff)), determined bythe calibration measurement, of the first quotient P′_(DC) and of theroot-mean-square value P′_(ACeff) of the periodically fluctuating secondquotient P′_(AC).

The first quotient P′_(DC) corresponds to the direct portion P_(DC) ofthe derived quantity P according to FIG. 1, and the periodicallyfluctuating quotient P′_(AC) corresponds to the alternating portionP_(AC) of this derived quantity P.

For the formation of the first quotient P′_(DC), the means 16 accordingto FIG. 2 is provided, while the second quotient P′_(AC) and theroot-mean-square value P_(ACeff) of this second quotient P′_(AC) areformed by the means 17.

Similar to the means 12 in FIG. 1, the means 17 can for example consistof a means 17 ₁, for the formation of the second quotient P′_(AC) and ofa means 17 ₂ for the formation of the root-mean-square value P′_(ACeff)of the second quotient P′_(AC), whereby the second quotient P′_(AC) issupplied to the means 17 ₂.

The means 18 corresponds to the means 13 in FIG. 1, and the means 19corresponds to the means 14 in FIG. 1.

This means that the means 18 is supplied with the value belonging to asampling point of the first quotient P′_(DC) and the root-mean-squarevalue P′_(ACeff) belonging to this sampling point of the second quotientP′_(AC), and the means 18 outputs to the means 19 the functional valueof the predetermined function f(P′_(DC), P′_(ACeff)) belonging to thisvalue of the first quotient P′_(DC) to this root-mean-square valueP′_(ACeff) as the functional value of this function belonging to thissampling point, to which means 19 on the other hand the value belongingto this sampling point of the second quotient P′_(AC) is supplied, andwhich forms from this functional value and this value of the secondquotient P′_(AC) the temperature-coefficient-compensated output signalS′ belonging to this sampling point, and outputs this output signal.

The means 18 can contain a lookup table in which a function value of thefunction f is respectively allocated to each pair of values consistingof a value of the first quotient P′_(DC) and a root-mean-square valueP′_(ACeff) of the second quotient P′_(AC) The means 19 can also containa processor that calculates the functional values of the function fallocated to the various value pairs of a value of the first quotientP′_(DC) and a root-mean-square value P′_(ACeff) of the second quotientP′_(AC), for example approximated with the aid of a polynomial of aselectable order.

An essential feature of the inventive method is the inclusion also ofthe root-mean-square value PACeff of the alternating portion P_(AC) or,respectively, of the root-mean-square value P′_(ACeff) of the secondquotient P′_(AC), and not only of the direct portion P_(DC) or,respectively, first quotient P′_(DC).

As is known, given optical adjusting P_(DC) is a good measure for thesensor head temperature and thus for its momentary current sensitivityor, respectively, voltage sensitivity, a misadjustment between theanalyzer and the polarizer causes however a dependence of the directportion P_(DC) or, respectively, first quotient P′_(DC) on the fieldstrength, whereby the previous compensation of the temperaturecoefficient fails. In contrast, in the inventive compensation a fieldstrength dependence of the direct portion P_(DC) or, respectively, firstquotient P′_(DC) is taken into account, and the temperature coefficientcompensation works even given poor adjustment between analyzer andpolarizer.

FIG. 3 shows, for example, the percent deviation of thenon-temperature-coefficient-compensated derived quantity P according toFIG. 1 from its ideal value without linear double refraction δ, whichlikewise influences the light polarization and is caused by mechanicaltensions, in particular given temperature fluctuations. The percentdeviation is plotted against the circular double refraction 2ρ/°, whichis directly proportional to the magnetic field strength, and against thelinear double refraction δ/°, whose change with temperature essentiallycauses the temperature coefficient of the derived quantity P. Ifanalyzer and polarizer are not adjusted precisely in relation to oneanother, the high measurement errors occurring here cannot be reduced bythe application of the known temperature coefficient compensationmethods. Given a de-adjustment of for example 5° between analyzer andpolarizer, the compensated signal hardly differs from the uncompensatedsignal.

FIG. 4 shows, in the same type of representation as FIG. 3, the percentdeviation of the signal S or, respectively, S′, corrected with theinventive method, from its ideal value without linear double refraction,given an angular misadjustment of 5° between analyzer and polarizer. Athird-order polynomial was used for the function f, to be determined bymeans of calibration measurement. The percent error remains for the mostpart below 0.5%. The error actually to be expected during a measurementis much smaller, because it is not be expected that the linear doublerefraction δ will change with temperature over such a large range.Fluctuations of the Verdet constant, which is decisive for the Faradayeffect, of the optical medium of the sensor with temperature arelikewise compensated given determination of the adaptation function f bymeans of calibration measurement. The precision can be further increasedby selection of an adaptation function f in the form of a polynomial ofa still higher order.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

I claim:
 1. A method for obtaining a temperature-coefficient-compensated output signal in an optical sensor for measuring a periodically fluctuating electrical and/or magnetic field strength, comprising the steps of: sending light that can be influenced by a field strength through the periodically fluctuating field strength, producing two intensity signals from said light that has been sent through and influenced in said sending step, said two intensity signals being separate from one another and being of intensities containing intensity portions that fluctuate in chronologically periodic fashion in phase opposition to one another dependent on the periodically fluctuating field strength, and deriving a quantity from the two intensity signals said quantity corresponding to a quotient of a difference of intensities of the two intensity signals and a sum of said intensities of said two intensity signals said quantity including a direct portion and a periodically fluctuating alternating portion with a root-mean-square value obtaining a temperature-coefficient-compensated output signal by formation of a quotient of the periodically fluctuating alternating portion of said quantity and a function determined by a calibration measurement of the direct portion and of a root-mean-square value of the periodically fluctuating alternating portion of said quantity.
 2. A method according to claim 1, further comprising the steps of: approximating the function determined by the calibration measurement by a polynomial of predeterminable degree.
 3. A method according to claim 1, further comprising the steps of: storing the function determined by the calibration measurement in approximated form in a lookup table.
 4. An apparatus for obtaining a temperature-coefficient-compensated output signal in an optical sensor for measuring a periodically fluctuating electrical and/or magnetic field strength, comprising: a means for obtaining a direct portion of a derived quantity, a means for obtaining an alternating portion of the derived quantity and formation of a root-mean-square value of said alternating portion, and a means for forming a quotient of the alternating portion of the derived quantity and a function of the direct portion and of the root-mean-square value of the alternating portion of the derived quantity. 